Ferric enterobactin receptor (FepA) is an E. coli outer membrane protein which functions to find an iron-chelator complex, ferric enterobactin, and transport it through the outer membrane. FepA is a member of a class of outer membrane transporters which recognizes their substrates with high specificity and catalyze active transport across the outer membrane. No structure exists for any member of this class of transporters. Because the protein contains no free cysteines, five cysteine mutants have been created by site-directed mutagenesis to aid derivatization with heavy atoms.